1. Field of the Invention
The present invention relates to a steering device equipped with an axially telescopic shaft constituted by an upper shaft with an outer spline formed therein and a lower shaft with an inner spline formed therein, and more particularly to a steering device having a telescopic function which is advantageous for vehicles, such as cab-over trucks, in which the entire cabin (cab) is rotated and power components as represented by an engine are exposed to the outside.
2. Description of the Related Art
There are a large number of cab-over trucks. In such trucks, the vehicle cab is opened and closed by rotation about the front lower end thereof as a rotation center, so as to expose the power components as represented by the engine to the outside, and, for example, the inspection of equipment is performed. When the cab is rotated and, for example, the inspection is performed, the entire steering device is rotated together with the cab.
The lower end portion of the steering device is connected to the steering unit of the front wheels, and when the cab is rotated and opened, a telescopic portion provided at part of the shaft constituting the steering device moves telescopically along the axial direction. An example of such steering device is disclosed in Japanese Patent Application Publication No. H6-241238.
The contents of Japanese Patent Application Publication No. H6-241238 are briefly described below. The reference numerals used herein are those of Japanese Patent Application Publication No. H6-241238. Initially, a spline tube 9 having a female spline on the inner circumferential surface and a spline shaft 10 having a male spline groove on the outer circumferential surface are engaged with each other. A sealing member 19 produced from an elastic material such as a rubber is externally fitted and fixed to the upper end portion of the spline tube 9, and the sealing member 19 maintains a water-tight state between the inner circumferential edge of the upper end opening of the spline tube 9 and the outer circumferential surface of the intermediate portion of the spline shaft 10.
The sealing member 19 has a support tubular portion 23 that can be externally fitted on and supported by the upper end portion of the spline tube 9 and a bent portion 24 of an inward flange shape that is provided at the upper end of the support tubular portion 23 and formed by bending diametrically inward while protruding upward from the upper end edge of the spline tube 9. First sealing lips 29 are formed at a portion that is slightly above the lower end edge of the support tubular portion 23 at the inner circumferential surface of the support tubular portion 23.
The first sealing lips 29 are of a comparatively small thickness and low rigidity. Therefore, when a difference in pressure occurs between the two sides of the first sealing lips 29, they are elastically deformed in a comparatively easy manner and the air freely circulates through third gaps 28. In the sealing member 19, a second sealing lip 30 is formed at the inner circumferential edge of the bent portion 24.
The inner circumferential edge of the second sealing lip 30 is in contact with the outer circumferential surface of the spline shaft 10 over the entire circumference. The second sealing lip 30 is of the so-called self-sealing type, and the inner circumferential edge thereof is pressed against the outer circumferential surface of the spline shaft 10 by the pressure of the fluid which is to be sealed. Therefore, the second sealing lip 30 is less likely to deform in the direction in which the seal is broken by a pressure change than the first sealing lip 29.
When, for example, muddy water that has adhered to the outer circumferential surface of the spline shaft 10 flows down along the outer circumferential surface, the second sealing lip 30 blocks this flow and the muddy water is prevented from penetrating into the spline tube 9. Where the shaft is telescopically moved in order to, for example, adjust the height position of a steering wheel 1, the air pressure inside the spline tube 9 changes.
For example, when the steering wheel 1 is lowered and the shaft is contracted, the pressure inside the spline tube 9 rises. This pressure is applied to the upper surface of the first sealing lips 29 through the first, second, and third gaps 20, 25, 28, causes the first sealing lips 29 to deform elastically downward, and separates the inner circumferential edge of the first sealing lips 29 from the bottom surface of valley portions 27.
As a result, the inside and outside of the spline tube 9 communicate with each other through the first, second, and third gaps 20, 25, 28, and the air located inside the spline tube 9 is discharged. Conversely, where the steering wheel 1 is raised and the shaft is extended, the pressure inside the spline tube 9 decreases.
Under the effect of the atmospheric pressure applied to the lower surface of the first sealing lips 29, the first sealing lips 29 are elastically deformed upward, the inner circumferential edges of the first sealing lips 29 are separated from the bottom surface of the valley portions 27, and the external air is introduced into the spline tube 9.